The invention relates to a method for tracking a locally generated spread spectrum signal sequence in order to synchronize the locally generated spread spectrum signal sequence with a corresponding received spread spectrum signal sequence. The invention also relates to a configuration for tracking a locally generated spread spectrum signal sequence.
The method according to the invention can be used with basic spread spectrum methods, such as direct sequence methods. In these methods, a message is modulated with a higher-frequency pseudo-random binary sequence before being transmitted. The receiver can extract the message from the pseudo noise signal produced if the receiver knows the binary sequence.
These methods are used in data communications, position finding and navigation. An important field of use is, e.g. the real-time satellite navigation according to the NAVSTAR GPS system (Navigation System with Timing And Ranging, Global Positioning System), which is described, e.g. by Schrxc3x6dter, in xe2x80x9cGPS Satellite Navigation,xe2x80x9d Franzis Verlag, Munich, 1994. For a receiver to be able to identify a transmitter and evaluate its information, it must know a transmitter-specific code (gold code) which is transmitted as a periodic signal sequence of predetermined length. In order to make the code available, all the codes of the transmitters in question are stored in the receiver. Since, furthermore the phase angle of each incoming signal is not known, the match between the received spread spectrum signal sequence and the locally generated spread spectrum signal sequence is established in an acquisition method. The comparison is performed by the correlation function of the signal which becomes almost one when there is synchronization.
Since the transmitter and the receiver are moving relative to one another, the synchronization must be subsequently maintained by correcting the phase of the locally generated spread spectrum signal sequence to the received spread spectrum signal sequence. This is done by regulating the clock of the locally generated spread spectrum signal sequence as a function of the phase deviation found.
It is known to use a so-called delay locked loop (DLL) for this purpose as is described, for example, in Holmes: Coherent Spread Spectrum Systems, Robert E. Krieger, 1990. This method is based on the locally generated spread spectrum signal sequence being phase shifted by the same amount preceding and following the expected punctual times and the received spread spectrum signal sequence being correlated with the spread spectrum signal sequences of this earlier and later time. The results are then subtracted from one another in order to receive the final correlation result.
In this method, it is found that a small phase shift is advantageous to the control characteristic with respect to the noise but that it is disadvantageous in a multiplicity of operating conditions when a good correction of the locally generated spread spectrum signal sequence is to be achieved. The smaller the early/late interval of the closed-loop control, the poorer becomes the dynamic behavior because the tracking can be lost with fast and sudden movements.
In U.S. Pat. No. 5,734,674 to Fenton et al., a method for synchronizing a locally generated spread spectrum signal sequence with a corresponding received spread spectrum signal sequence is described. In this method, a large early/late interval is initially used which covers a large capture range.
In the course of the closed-loop control, this large early/late interval is narrowed down. The actual signal tracking is then done with a small early/late interval. In a very dynamic scenario or with a very noisy initial signal, the closed-loop control may, therefore, lose lock.
It is accordingly an object of the invention to provide a method and a configuration for tracking which overcome the above-mentioned disadvantages of the heretofore-known methods and configurations of this general type and which improve the dynamic behavior and, at the same time improve a variance of the distributed results.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of tracking a locally generated spread spectrum signal sequence, the method includes the steps of:
correlating a received spread spectrum signal sequence with a locally generated spread spectrum signal sequence with respect to a synchronization time at a relatively earlier time and a relatively later time in an early/late interval;
subtracting correlation responses from one another;
controlling a clock of the locally generated spread spectrum signal sequence as a function of a result of the subtracting step for synchronizing the locally generated spread spectrum signal sequence with a received spread spectrum signal sequence corresponding to the locally generated spread spectrum signal sequence;
performing correlations based on at least two hierarchically staggered closed-loop controls with different early/late intervals;
providing an early interval of a first one of the at least two hierarchically staggered closed-loop controls having a relatively smaller early/late interval such that the early interval of the first one of the at least two hierarchically staggered closed-loop controls does not become earlier than an early interval of a second one of the at least two hierarchically staggered closed-loop controls having a relatively next-larger early/late interval; and
providing a late interval of the first one of the at least two hierarchically staggered closed-loop controls such that the late interval of the first one of the at least two hierarchically staggered closed-loop controls does not become later than a late interval of the second one of the at least two hierarchically staggered closed-loop controls.
In other words, the object of the invention is achieved by a method for correlating a locally generated spread spectrum signal sequence for synchronization with a corresponding received spread spectrum signal sequence, wherein the latter is correlated with the locally generated spread spectrum signal sequence with respect to the synchronization time at an earlier and a later time in an early/late interval, the correlation responses are subtracted from one another and the clock of the locally generated spread spectrum signal sequence is controlled as a function of the result of the subtraction, wherein correlations are performed through the use of at least two hierarchically staggered closed-loop controls with different early/late intervals and in that the early interval of one closed-loop control having a small early/late interval must not become earlier than the early interval of the other closed-loop control having a next-larger early/late interval and, furthermore, the late intervals of the one closed-loop control having a small early/late interval must not become later than the late interval of the other closed-loop control having a next-larger early/late interval.
According to another mode of the invention, the correlations are performed by starting with a largest one of the early/late intervals; and the correlations are performed with a given one of the early/late intervals smaller than the largest one of the early/late intervals subsequent to reaching a control limit of the largest one of the early/late intervals.
According to yet another mode of the invention, an early time and a late time of a given one of the at least two hierarchically staggered closed-loop controls having a smallest one of the early/late intervals are used for calculating a punctual time.
With the objects of the invention in view there is also provided, a configuration for tracking a locally generated spread spectrum signal sequence, including:
a delay locked loop configuration including filter devices, the delay locked loop configuration comparing locally generated early spread spectrum signal sequences, locally generated punctual spread spectrum signal sequences and locally generated late spread spectrum signal sequences with a received spread spectrum signal sequence;
the filter devices phase-shifting the locally generated early spread spectrum signal sequences and the locally generated late spread spectrum signal sequences with respect to the locally generated punctual spread spectrum signal sequences;
at least two control loops having different early/late intervals and having respective inputs, the at least two control loops being connected in parallel at the respective inputs;
the at least two control loops including a first control loop and a second control loop, the first control loop having a first output and having a relatively smaller early/late interval, the second control loop having a second output and having a relatively next-larger early/late interval;
a comparator, connected to the first output and to the second output, for comparing output signals from the first control loop and the second control loop; and
a data processing unit connected to the comparator.
In other words, the object of the invention is achieved by a DLL configuration which performs the method according to the invention wherein at least two control loops having different early/late intervals are connected in parallel at their respective inputs, wherein the output of a control loop having a smaller early/late interval and the output of the control loop having the in each case next-larger early/late interval are connected to a comparator which compares the output signals of these two control loops, and wherein all comparators are connected to a data processing unit which ensures, based on the output signals of all control loops, that the associated limit values of the early/late intervals are not exceeded.
Accordingly, a basic concept of the invention is based on the idea of allowing a number of DLL configurations to act together in a hierarchical structure. The individual closed-loop controls have phase shifts in such a manner that they are interleaved and initially the closed-loop control having the largest early/late interval is operated. When changing over to the DLL with the next smaller early/late interval, the closed-loop control must only be within the limit values of the DLL having the next-larger early/late interval etc. Graphically expressed, the method can be compared with carriages which are nested together where a smaller carriage can in each case move on the larger carriage. The smallest carriage makes its own movement, on the one hand and, on the other hand, it also participates in the movements of the other carriages when it reaches the limit of its own range of movement.
The invention has the advantage that two fundamentally incompatible conditions can be met, namely good noise suppression, on the one hand, and good dynamic behavior, on the other hand.
It is especially advantageous if there are four control loops. This makes it possible to achieve a particularly good suppression of the variance in spread.
According to another feature of the invention, further comparators are connected to respective pairs of the control loops.
According to yet another feature of the invention, the at least two control loops are second-order control loops.
According to a further feature of the invention, the first control loop and the second control loop are configured such that a relation xcex94kxe2x89xa7xcex94kxe2x88x921/2 is met, where 2xcex94k is the relatively smaller early/late interval and 2xcex94kxe2x88x921 is the relatively next-larger early/late interval.
According to another feature of the invention, a given one of the at least two control loops has a relatively largest early/late interval such that a relation 2xcex941=Tc is met where 2xcex941 is the relatively largest early/late interval and Tc is a chip period of one of the spread spectrum signal sequences.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a tracking method and a configuration for carrying out the method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.